Traditionally vehicle wheel alignment has been made in relation to a physical reference of the frame passing through the supposed center of the steerable wheels which are usually the front wheels. The difficulty with the traditional alignment theory is that the actual heading of the vehicle is not taken into consideration. It is known that makers of vehicle wheel alignment equipment generally call for preliminary rear wheel, non-steerable wheel, alignment to insure proper alignment of the steerable front wheels. However, it is difficult to align the non-steerable wheels as they are usually fixed which complicates the correction if required. In most wheel alignment shops and service facilities there is no provision made, or equipment provided, for physically relocating the position of the non-steerable wheels which are fixed by the manufacturer.
Examples of the prior art include Holub U.S. Pat. No. 3,337,961 issued Aug. 29, 1967; MacMillan U.S. Pat. No. 3,091,862 issued June 4, 1963; Manlove U.S. Pat. No. 3,164,910 issued Jan. 12, 1965; and Senften U.S. Pat. No. 3,782,831 issued Jan. 1, 1974. In the disclosure of Holub the apparatus comprise light projectors for each of a pair of wheels mounted so the light beam of each projector can be directed longitudinally as well as transversely. The angular orientation of the projected light beams is shown by viewing a scale so that all wheel alignment characteristics are required to be remembered by the operator without any way of automatically compensating the alignment of one set of wheels by the alignment of the other set of wheels. In MacMillan the device is limited to line of sight means and the disclosure recognizes only that the rear wheels follow the front wheels in paths symmetrically parallel to the front wheels. This relationship is not always true so that the mis-alignment of rear wheels is not considered at all. In Manlove the apparatus is designed primarily to check front wheel alignment by reading scales, but rear wheel track may also be checked using the same reading scales so that compensating information is not stored but must be remembered by the operator. In Senften the apparatus differs from the foregoing in that it is capable of electronically finding front wheel position relative to a reference which, while in no way associated with the alignment of the rear wheels, is a more accurate way of determining wheel position than by the use of mechanical scales.
In addition to the issued patents there is the pending application of Hunter, Ser. No. 675,347, filed Apr. 9, 1976, directed to wheel alignment apparatus and a method which does not involve means to automatically determine the front wheel alignment by information about the rear wheel alignment.
It is easily understood from these prior art disclosures that none is concerned with, nor provides apparatus for finding information about the alignment characteristics of non-steerable wheels and storing that information in such a way that it is useful in aligning the steerable wheels.